Magnetic resonance systems require static magnetic fields that are relatively strong, homogeneous and stable. It appears as if the best magnets for providing such fields are superconducting magnets. The magnet arrangement for MR systems require a coil system consisting of at least one circular cylindrical field coil to provide the large static homogenious field. In magnetic resonance imaging (MRI) systems at least one additional gradient coil is required to enable the location of the free induction decay (FID) signals that are used for imaging. In most present day systems, in fact, three gradient field coils are used. In addition radio frequency transmission and receiving coils are wound in the magnet. The receiving and transmitting coils can be a single coil.
The superconducting arrangement in the past generally has included a vacuum vessel or dewar container for preserving a vacuum and aiding in maintaining the low temperatures required for superconducting. In the dewar, there is provided a liquid helium container. The above enumerated magnetic coils are placed within the liquid helium to obtain superconductivity.
In the systems presently used the magnet system is a cylindrical arrangement having an internal central bore which receives the patient for imaging or spectograph analysis or both. In the general coil magnetic arrangement used for MR systems, the helium tank is surrounded by a gas cooled shield. Around the gas cooled shield is a nitrogen shield which extends from a liquid nitrogen container which is also located within the dewar and is radially external to the helium tank. Both tanks are located within the dewar between the bore and the radial outer dimension of the dewar.
In the prior art, the dewar has been made sufficiently strong to both act as the vacuum vessel and to support the magnetic system including the liquid nitrogen and liquid helium tanks and the coils arranged within the liquid helium tank. The prior art dewars have been strengthened by incorporating a frame onto the vacuum vessel and adding to the heft of the outer shell of the dewar. The prior art arrangement has thus required a relatively large mummber of welding operations to incorporate the frame and the vacuum vessel. The welding operations of course increase the costs of the cryostats well as decrease the reliability of the systems since welding joints are susceptable to leaks. Also, because of the strength needed to support the magnetic system in the past, it has been necessary to construct the vacuum vessels from relatively strong and expensive metals.
Accordingly, it is an object of scientists working in the field to find the means and methods for providing cryomagnetic arrangements for MRI systems wherein the vacuum vessel may be constructed of materials other than expensive and hard to work metals and in any event to be more reliable due to having less welds.